FADING SIMULATOR

Abstract: A fading simulator for generating multipath-faded test signals which have attenuation notches of precisely controllable frequency position and attenuation depth from a modulated input signal of known bandwith. The input signal is fed to a splitter (10) that directs two in-phase components to a first path (14) and a second path (18). The input to each path is divided by a splitter (22) into in-phase and quadrature sub-signals (24 and 26) having a quadrature phase relationship, each sub-signal being fed to a respective inverting attenuator (28 and 32), the outputs of the attenuators then being combined in a combiner (36) and the combined output being passed through a variable delay line (16) to generate the output of the respective path. The outputs of each path are then combined in a combiner circuit (12) to generate the multipath-faded test signal. All four inverting attenuators are preferably substantially identical to one another so that the two paths have close to identical temperature-drift and other characteristics. The attenuators and delay lines are electronically controlled in tandem, preferably using a microprocessor and stored program, to generate precision, static or dynamic, attenuation notches desired.

Description: TITLE: FADING SIMULATOR TECHNICAL FIELD This invention relates to electronic fading simulators and simulation methods for use in simulating the short-term fading of radio signals due to multi-path interference phenomena. Such simulators may be used to check the ability of radio receiving equipment, such as high-capacity digital microwave receivers, to handle various types and patterns of signal fades with respect to data loss. BACKGROUND TO THE INVENTION The background to the present invention is set out in the specification of prior PCT patent application No. PCT/AU89/00254 filed in the name of the of the present applicant (herein called Our prior application'). Indeed, our prior application represents the closest prior art known to the applicant relevant to the present invention. Reference should be made to the specification of that application for a discussion of the technical problems faced in the design and operation of fading simulators and for an explanation of the applicant's desiderata in designing such devices. The simulator of our prior application allowed microprocessor-controlled calibration of notch depth and notch position as independent variables thereby greatly reducing calibration time. Since the drift of component parameters normally requires frequent re-calibration of the simulator during use, the significant reduction in calibration time offered by the simulator design and method of our prior application provided a substantial advantage with respect to the art at the time it was filed. Nevertheless, calibration of that simulator could still take a few minutes and it would clearly be desirable if such calibrations were to take much less time while still allowing accurate and predictable simulation of deep signal fades which sweep rapidly across the signal spectrum. OBJECTIVES OF THE INVENTION It is therefore the object of the invention to provide a fading-simulator and method which enable shorter calibration times and/or longer times between calibrations without sacrificing precision or fading performance with respect to simulators of the prior art. OUTLINE OF INVENTION This invention is based upon the realisation that, if the two parallel signal paths in the simulator can be substantially identical, the differential drift of the parameters of the components in these paths could be significantly reduced so that fewer calibration points are needed, more rapid calibration of the simulator should be possible and/or greater stability over a longer period would be obtained. Such an approach is, however, counter-intuitive as it involves adding many 'unnecessary' components to the signal path that is not normally deliberately phase-shifted and, therefore, increasing the complexity of the circuit and, superficially, the time required for calibration. Moreover, to be effective, the 'unnecessary' components in the path which is not normally phase-shifted in a controlled manner, must now be controlled in tandem with the components in the control-path, a requirement which intuitively could be expected to result in delays and inaccuracy. Nevertheless, it will be shown that the use of a symmetrical simulator (that is, one having parallel paths that are similar and, preferably, substantially identical) offers substantial advantages. From one aspect, therefore, the invention comprises a method of generating a multipath-faded test signal, having an attenuation notch of controllable frequency position and/or attenuation depth, from a modulated input signal of known bandwidth, comprising the steps of: feeding the input signal through two parallel paths having similar (preferably, substantially identical) electronically-controllable complex attenuators in each path, combining the signals from each path to generate a multipath-faded test signal, and jointly controlling the complex attenuators to generate the desired attenuation notch within the test signal. Normally, the modulated input signal will be passed through a splitter circuit which feeds each of the two paths and the outputs from the attenuator means will be mixed in a combiner circuit to generated the faded test signal. The two outputs of the splitter may be in-phase or in quadrature (ie; have a 90° phase difference). In one possible configuration, each of the attenuator means may comprise a pair of inverting attenuators arranged in parallel, the inputs of each pair of inverting attenuators being derived from a quadrature splitter and the outputs of each pair being mixed by a combiner circuit. In this configuration, the outputs of the splitter circuit which feeds the two paths need not be phase-shifted relative to each other. In another possible configuration, the outputs of the splitter circuit which feeds the two paths may be phase-shifted with respect to one another and, if so, a quadrature (90°) shift is preferred. In that case, each path may comprise a further splitter having one output which feeds an inverting attenuator that, in turn, feeds one input of a further combiner before the two paths are re-joined (by combing the outputs of the further combiners) by means of the first-mentioned combiner circuit. The two outputs of each of the two further splitters need not be phase-shifted with respect to one another. The second output of each of the further splitters is then preferably fed via a respective further inverting attenuator to the second input of the further combiner in the other path. From another aspect, therefore, the present invention comprises a fading simulator for generating multipath-faded test signals, having attenuation notches of controllable frequency position and/or attenuation depth, from a modulated input signal of known bandwidth, comprising: a first signal path and a second signal path extending in parallel between input circuit means at which the paths diverge and output circuit means at which the paths converge, first electronically-controllable complex attenuator means connected in the first path for effecting the dynamic attenuation and phase- shifting of the signal in that path, second electronically-controllable complex attenuator means substantially identical or similar to the first attenuator means connected in the second path for effecting the dynamic attenuation and phase-shifting of the signal in that path, and electronic control means connected to each complex attenuator for controlling them in tandem and in accordance with a stored program to generate an output multipath dynamically-faded test signal determined by the program. It will be appreciated that fading simulators of the type indicated are of balanced or symmetrical form so that component drift in one arm or path will be compensated by much the same drift in the same components in another complementary arm or path. This enables much greater precision in calibration, much longer times between successive recalibrations and/or much fewer calibration points to achieve a satisfactory accuracy operation. Note again that calibration involves the use of microprocessor controls and methods substantially as described in our above-mentioned prior patent application. DESCRIPTION OF EXAMPLES Having broadly portrayed the nature of the present invention, particular embodiments will now be described by way of example and illustration only. In the following description, reference will be made to the accompanying drawings in which: Figure 1 is a block diagram of a first example of a balanced or symmetrical fading simulator which can be built from commercially available components in accordance with the principles of the present invention. Figure 2 is a block diagram of a second example of a balanced or symmetrical fading simulator which can be built from commercially available components in accordance with the principles of the present invention. The fading simulator of both embodiments of the invention are suitable for use in digital radio analyser instruments for testing high capacity microwave links (eg, 256QAM) being capable of generating maximum notch depths of in excess of 65 dB, attenuation steps of better than 0.5%, frequency precision of better than 0.05% bandwidth, and notch-sweep rates of up to 4000 MHz/s. The general principles of the operation and control of the simulator and the functions of the associated digital radio analysers are as disclosed in our prior patent application. Referring now more particularly to the embodiment shown in Figure 1 , a modulated input signal having a bandwidth of, say, 45 MHz and a median frequency of, say, 75 MHz is fed into a 'Magic-T' splitter 10 which generates two substantially identical outputs that are fed through two substantially identical parallel paths (Path A and Path B) to be re-combined in Magic-T combiner 12 to form the desired faded test-signal output. Path A comprises a complex attenuator circuit 14 in series with a controllable delay-line 16, while Path B comprises an identical complex attenuator circuit 18 in series with a delay-line 20 that is essentially the same as delay line 16 except that it need not be controllable. That is, delay-line 20 may be of a fixed length while delay- line 16 may be switchable to connect two or more different lengths of line in-circuit. Such a switchable delay-line was described in our prior patent application. Since the complex attenuator circuits 14 and 18 are essentially identical, only the components of attenuator 14 will be described. Attenuator circuit 14 comprises a quadrature hybrid splitter 22 which generates an un-phaseshifted output signal ("O Degrees") on line 24 and a 90° phase-shifted output signal ("90 Degrees) on line 26. The signal on line 24 is fed to a first inverting attenuator 28 which may be controlled by means of a unit 30 to vary the amplitude of the in-phase component in Path A (hence the label "A.I" on unit 30). The quadrature signal on line 26 is fed to a second inverting attenuator 32 which is arranged in parallel with attenuator 28 and is substantially identical thereto. Attenuator 32 is controlled by unit 34 so as to vary the amplitude of the quadrature signal in Path A (hence the label "A.Q" on unit 34). Finally the in-phase and quadrature outputs of attenuators 28 and 32 are combined in a Magic-T arranged as combiner circuit 36. It will be appreciated that the output of each of the complex attenuator circuits (14 and 18) will be the input signal to the simulator with its amplitude attenuated by a controlled amount and its phase at any angle within 2π radians. Thus after combining the signals from the two paths A and B by means of combiner 12, the output of the simulator is the modulated input with a superimposed fade-notch. By controlling the four inverting attenuators (together with delay-line 16) in tandem, the notch can be made to become deeper or more shallow, wider or more narrow and to stay at a fixed frequency or to sweep rapidly or slowly within the bandwidth. Because the components in each path are substantially identical, they will have substantially identical temperature and control characteristics, making possible high-precision control with minimal calibration overhead. Referring now more particularly to the second embodiment as shown in Figure 2, it will be seen that a symmetrical multi-path fading-simulator circuit is also involved. This circuit functions in an analogous fashion to that of the first embodiment to produce essentially the same results. It has been found that the second embodiment can be implemented more easily than the first, if microprocessor-driven controls of the type generally indicated in our prior patent application are employed. The modulated input signal is fed into a quadrature hybrid splitter 112 which simply generates two outputs, one (on line 114) comprising the un-phaseshifted input signal ("0°") and the other (on line 116) comprising the input signal with a 90° phase shift. These outputs are fed to Magic-T splitters 118 and 120 (respectively) the outputs of which are not phase-shifted again relative to one another. The outputs 122 and 124 of Magic-T 18 are passed to inverting attenuators 26 and 128 respectively, while outputs 130 and 132 of Magic-T 120 are passed to inverting attenuators 134 and 136 respectively. The outputs of attenuators 126 and 134 are mixed in another Magic-T circuit 138 configured as a combiner rather than a splitter, the outputs of attenuators 128 and 136 similarly being combined in a Magic-T 140. The output of Magic-T/combiner 138 is fed via a controllable delay-line 142 to one input (144) of an output Magic-T/combiner 146 circuit, while the output of Magic- T/combiner 140 is fed via a fixed delay-line 148 via line 150 to the other input of output Magic-T/combiner 146. Most of the circuit elements of both embodiments blocks can conveniently be formed from commercially available hybrid circuits; for example, from Minicircuits of Brooklyn, NY. Thus: Circuit Block No. Minicircuits Type No. Figure 1 22 PSCQ-2-90 10,36,12 TSC-2-1 2288,,3322,, PAS-3 Figure 2 112 PSCQ-2-90 118,120,138,140,146 TSC-2-1 126,128,134,136 PAS-3 It will be appreciated from the above that the fading simulator can operated substantially as described in our prior patent application in order to generate the desired faded test-signals by suitably controlling the four attenuators in tandem. However, it has been found that, despite nearly doubling the number of basic circuit elements in the simulator, the stability and precision obtainable are of an order of magnitude better than that obtained from simulators made in accordance with our prior patent application. The reason for this lies essentially in the reduction of differential parameter drift between the two arms or paths of the simulator with changing temperature. It will also be appreciated that the examples of the invention described above meet the objects and advantages set out at the beginning of this specification. However, those skilled in the art will also understand that many variations and modifications can be made to the invention as disclosed without departing from its spirit or scope. SUBSTITUTE SHEET {RULE 26)

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1 A fading simulator for generating multipath-faded test signals, having attenuation notches of controllable frequency position and/or attenuation depth, from a modulated input signal of known bandwidth, comprising: a first signal path and a second signal path extending in parallel between input circuit means at which the paths diverge and output circuit means at which the paths converge, first electronically-controllable complex attenuator means connected in said first path for effecting the dynamic attenuation and phase-shifting of the signal in that path, second electronically-controllable complex attenuator means of substantially ide¬ ntical or similar circuit design to said first attenuator means connected in said second path for effecting the dynamic attenuation and phase-shifting of the signal in that path, and electronic control means connected to each complex attenuator for controlling said attenuators in tandem and in accordance with a stored program to generate an output multipath dynamically-faded test signal determined by said program. 2 A fading simulator according to claim 1 wherein: each of said complex attenuators comprises: an electronically-controllable primary inverting attenuator, an electronically controllable secondary inverting attenuator, and an attenuator combiner circuit connected to the output of each of said inverting attenuators in the respective path, the output of said combiner circuit comprising the output of the respective complex attenuator, and wherein said primary and secondary inverting attenuators of each complex attenuator are each connected to said input circuit means in such a manner that the phase of the input signal received by the primary inverting attenuator is in quadrature with respect to the phase of the input signal received by the secondary inverting attenuator. 3 A fading simulator according to claim 2 wherein: the input circuit means includes a signal splitter adapted to divide the modulated input signal into two substantially identical path-input signals having a quadrature phase relationship, the input circuit means is connected to feed said path input signals to respective ones of said signal paths, each of said complex attenuators includes path-splitter circuit means connected to receive the respective path-input signal from the said input circuit and to divide that path-input signal into two substantially identical inverting-attenuator input signals having the same phase, one path-input signal of each path-splitter circuit means is connected to feed one of its said inverting-attenuator input signals to one of the inverting-attenuators in its path and is connected to feed the other of its said inverting-attenuator input signals to one of the inverting attenuators in the other path. 4 A fading simulator according to claim 2 wherein: the input circuit means includes a signal splitter adapted to divide the modulated input signal into two substantially identical path-input signals having the same phase, the input circuit means is connected to feed said path-input signals to respective ones of said signal paths, each of said complex attenuators includes path-splitter circuit means connected to receive the respective path-input signal from the input circuit means and to divide that path-input signal into two substantially identical inverting-attenuator input signals having a quadrature phase relationship, and said path-splitter circuit of each path is connected to feed its inverting-attenuator input signals to respective ones of the inverting-attenuators in its respective path. 5 A fading simulator according any preceding claim wherein: a first delay-line is connected in the first path in series with said first attenuator means, a second delay-line is connected in the second path in series with said second attenuator means, and at least one of said delay lines is electronically controllable, the said controllable delay line being connected to said control means for control thereby. 6 A fading simulator according to claim any preceding claim wherein: said control means includes a temperature sensor for sensing the average operating temperature of the simulator or of each complex attenuator and for varying the control of said attenuators as a function of said temperature or temperatures. 7 A fading simulator for generating output multipath-faded test signals, having attenuation notches of controllable frequency position and/or attenuation depth, from a modulated input signal of known bandwidth, substantially as herein-before described with reference to Figure 1 or Figure 2. 8 A method of generating a multipath-faded test signal, having an attenuation notch of controllable frequency position and/or attenuation depth, from a modulated input signal of known bandwidth, comprising the steps of: feeding the input signal through two parallel paths having similar (preferably, substantially identical) electronically-controllable complex attenuators in each path, combining the signals from each path to generate a multipath-faded test signal, and jointly controlling said complex attenuators to generate the desired attenuation notch within said test signal. 9 A method of generating a multipath-faded test signal having an attenuation notch of controllable frequency position and/or attenuation depth, by feeding a modulated input signal of known bandwidth through two parallel paths and combining the outputs of the paths to generate the test signal, the method comprising the steps of: phase shifting the input signals to the parallel paths so that they are in quadrature, feeding the input signal of one path to first and second controllable inverting attenuators, feeding the input signal of the other path to third and fourth controllable inverting attenuators, all of said attenuators being of substantially identical or similar design, combining the outputs of the first and third attenuators to generate the output of the first path, combining the outputs of the second and fourth attenuators to generate the output of the second path, ^^^ ^ (RULE ^ and controlling all attenuators in tandem to generate the desired test signal. 10 A method of generating a multipath-faded test signal having an attenuation notch of controllable frequency position and/or attenuation depth, by feeding a modulated input signal of known bandwidth through two parallel paths and combining the outputs of the paths to generate the test signal, the method comprising the steps of: directing the input signals to the parallel paths so that they are in-phase, splitting the input signal to each path into in-phase and quadrature sub-signals, feeding the sub-signals of the first path to respective first and second controllable inverting attenuators and combining the outputs of said attenuators to form the output of the first path, feeding said sub-signals of the second path to respective third and fourth controllable inverting attenuators and combining the outputs of said third and fourth attenuators to form the output of the second path, and controlling all attenuators in tandem to generate the desired test signal.

Inventors: BEARD DAVID LEWIS, MARTIN ANDREW LOUIS, BEARD, DAVID, LEWIS, MARTIN, ANDREW, LOUIS
Applicants: BEARD DAVID LEWIS, MARTIN ANDREW LOUIS, MARTIN COMMUNICATIONS PTY LTD, BEARD, DAVID, LEWIS, MARTIN COMMUNICATIONS PTY. LTD., MARTIN, ANDREW, LOUIS
Application year: 1994
Main classification: H04B17
IPC codes: G01R31, H04B1, H04B17